fix(federation): isolate Filigree MCP Python imports

[tachyon-beep]

Motivation

• Prevent an attacker-controlled repository from shadowing the installed filigree Python package when the local Filigree MCP subprocess is launched from the project root, which could enable arbitrary code execution during MCP tool calls.

Description

• Change the resolved Python MCP launcher to run the module in isolated mode by adding -I to the Python arguments so the analyzed project is not added to Python's import path (resolve_filigree_mcp_command now delegates to filigree_python_mcp_command).
• Preserve existing behavior by keeping the --project <root> argument when a project root is present.
• Add filigree_python_mcp_command helper to centralize the safe command construction and a regression test python_mcp_command_isolates_imports_from_project_root that verifies the program and args include the isolation flag.
• Only the launcher construction was changed; the fallback to the standalone filigree-mcp binary and the MCP subprocess-driving logic remain unchanged.

Testing

• Ran cargo fmt --check, which succeeded.
• Ran the new targeted unit test with cargo test -p loomweave-federation filigree::tests::python_mcp_command_isolates_imports_from_project_root -- --exact, which passed.
• Attempted cargo test -p loomweave-federation (full crate suite): the new regression and most tests ran, but several pre-existing HTTP mock tests failed or hung in the parallel full-suite run, so the full run was interrupted; the failures appear unrelated to the Python-launcher change and affect HTTP client/mock expectations.

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Pull request overview

This PR hardens the Filigree MCP launcher used by loomweave-federation by ensuring that when Filigree is launched via Python module execution, it runs in Python isolated mode to prevent project-root import shadowing of the installed filigree package.

Changes:

• Switch the Python-based MCP launcher path to use a centralized helper that adds python -I -m filigree.mcp_server.
• Preserve existing --project <root> behavior when a project root is available.
• Add a regression unit test ensuring the resolved Python command includes the -I isolation flag.

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[tachyon-beep]

Superseded by the rebase onto release/1.6.0 (the working release branch) — same commit, landing there. The CI failure here was an unrelated serve test.